In general, optical modulators vary optical signals for communication of the signals and are useful in a number of applications, including phased array radar, antenna remoting, optical processing of microwave signals and CATV signal distribution. A typical optical modulator has several layers of semiconducting material, including quantum wells, and is fabricated on a monolithic microwave integrated circuit (MMIC). An MMIC is formed by fabricating active and passive circuit elements onto or into the surface of a semi-insulating semiconducting substrate by various methods. These methods include deposition and etching schemes as well as utilizing photolithograph processes for pattern definition.
A quantum yell is defined as a layer of a material with a relatively small band gap between its conduction band and valence band interposed between two layers of material with larger band gaps acting as barriers. Specifically, the barrier layers are typically the same material and have a higher conduction band edge than the conduction band edge of the intermediate layer. In quantum well structures, a phenomenon called the quantum-confined Stark effect (QCSE) significantly alters the optical absorption and index of refraction of the materials when an electric field is applied. The QCSE "red shifts" the absorption spectrum at energies near the conduction band edge, i.e., the wavelength of the absorption spectrum changes toward a longer wavelength, and alters the refractive index for photon energies below the band edge. The modulation of optical signals is based on electroabsorption and electrorefraction in the modulator and the changes in absorption and refractive index are affected by specific properties of the quantum well.
J. E. Zucker et al., "Miniature Mach-Zehnder InGaAsP Quantum Well Waveguide Interferometer for 1.3 .mu.m", IEEE Photon. Tech. Lett., 2, 32, 1990 and E. Bigan et al., "Efficient Electroabsorption in InGaAsP/InGaAsP MWQ Optical Waveguide", Electron. Lett., 27, 1607, '91, the entire disclosures of which are incorporated herein by reference, show electrorefraction and electroabsorption modulators fabricated to include uncoupled quantum wells. These modulators are small in size and provide a potentially large bandwidth. Disadvantageously, these modulators require a high drive voltage and suffer from relatively high insertion loss. Insertion loss is the loss of optical power caused by the insertion of an optical component.
N. Debbar et al., "Coupled GaAs/AlGaAs Quantum Well Electroabsorption Modulators for Low Electric Field Optical Modulation", J. App. Phys., 65, 383, 1989 and K. Nakamura et al., "Numerical Analysis of the Absorption and the Refractive Index Change in Arbitrary Semiconductor Quantum-Well Structures", IEEE J. Quantum Electron., 28, 1670, '92 disclose symmetric and asymmetric coupled quantum wells for use in optical devices, respectively. The entire disclosures of these documents are also incorporated herein by reference.